This invention relates to an image forming apparatus for forming an image by using a photosensitive member such as a sheet with a surface coated with microcapsules encapsulating a colorless dye and a light-curable agent which hardens upon exposure to light and, more particularly, to an image forming apparatus which forms an image by exposing such a photosensitive sheet to image-forming light by means of a static exposure-type optical system.
Japanese Patent Publication Tokkai 58-88739 has disclosed a method of forming an image by using a photosensitive sheet with one surface of its sheet-like base material coated with microcapsules encapsulating a colorless dye of a specified type and a light-curable agent which hardens upon exposure to light. If such a photosensitive sheet is exposed to image-forming light such as reflected light from a document, only those of the microcapsules on the photosensitive sheet which have been exposed to light become hard selectively and a latent image corresponding to the exposed image is formed on the photosensitive sheet. If an image transfer sheet with a surface coated with a developing agent having a chromogenic effect of the colorless dye encapsulated in the microcapsules is superposed on such a photosensitive sheet with a latent image thus formed thereon and compressed together, those of the microcapsules which have not been hardened rupture, causing the colorless dye encapsulated therein to flow out thereof. The colorless dye which flows out of these ruptured microcapsules reacts with the developing agent on the image transfer sheet, forming a desired image on the image transfer sheet.
Image forming apparatus making use of a photoconductive body are also commonly known. With such an apparatus, an electrostatic image is formed by exposing a photoconductive body to image-forming light and, after toner is electrostatically attached to the electrostatic latent image, the toner image thus formed is transferred onto an image transfer sheet.
With such an image forming apparatus, reflected light from a document or transmitted light through a slide is used as the image-forming light for exposing a photosensitive sheet or a photoconductive body to form a latent image thereon. Such image-forming light is usually directed onto a photosensitive body either by a static exposure method or by a slit exposure method. The static exposure method is advantageous because it does not require complicated mechanisms for a scan by its optical system, while the slit exposure method does require such a scan. Thus, the control of the optical system is easier with the static exposure method. With the static exposure method, furthermore, the original document or slide has only to be exposed all at once for a specified length of time and this makes it possible to reduce the total time required for the exposure, compared with the slit exposure method whereby each part of an original must be exposed for a specified length of time. Thus, the overall processing time required for forming an image can be reduced by the static exposure method.
For such an image-forming exposure, use is generally made of light emitted from a source such as a tungsten lamp but such light sources deteriorate with time. The total amount of light may decrease and the spectroscopic distribution characteristic may change. When exposure is made by a static exposure method, the entire document or slide is exposed to light for a specified length of time. Although this presents no problem while the light source is still new and has not deteriorated yet, an underexposure may result if the light output from a deteriorated source is smaller but the same exposure time is used. An underexposed image is darker and its quality is low. When a full-color image is formed, a change in the spectroscopic distribution characteristic of the light source due to deterioration with time may adversely affect the color balance of the formed image. Light sources generally used for providing image-forming light usually become weak with time in the shorter wavelength region and images formed with such a deteriorated light source tend to be strong in magenta and cyanic.
In image forming apparatus, it is not only the light source that deteriorates with time. Optical components such as mirrors and lenses used for image formation become dirty and otherwise deteriorated, adversely affecting the reflectivity of the mirrors and the transmissivity of the lenses. For this reason, too, there tends to be an underexposure if an image is exposed for the same length of time and the result is a uniformly dark image.